NR4A nuclear receptors restrain B cell responses to antigen when second signals are absent or limiting.

Antigen stimulation (signal 1) triggers B cell proliferation and primes B cells to recruit, engage and respond to T cell help (signal 2). Failure to receive signal 2 within a defined time window results in B cell apoptosis, yet the mechanisms that enforce dependence on co-stimulation are incompletely understood. Nr4a1-3 encode a small family of orphan nuclear receptors that are rapidly induced by B cell antigen receptor stimulation. Here, we show that Nr4a1 and Nr4a3 play partially redundant roles to restrain B cell responses to antigen in the absence of co-stimulation and do so, in part, by repressing the expression of BATF and, consequently, MYC. The NR4A family also restrains B cell access to T cell help by repressing expression of the T cell chemokines CCL3 and CCL4, as well as CD86 and ICAM1. Such NR4A-mediated regulation plays a role specifically under conditions of competition for limiting T cell help.

Self-reactivity on a spectrum: A sliding scale of peripheral B cell tolerance.

Efficient mechanisms of central tolerance, including receptor editing and deletion, prevent highly self-reactive B cell receptors (BCRs) from populating the periphery. Despite this, modest self-reactivity persists in (and may even be actively selected into) the mature B cell repertoire. In this review, we discuss new insights into mechanisms of peripheral B cell tolerance that restrain mature B cells from mounting inappropriate responses to endogenous antigens, and place recent work into historical context. In particular, we discuss new findings that have arisen from application of a novel in vivo reporter of BCR signaling, Nur77-eGFP, expression of which scales with the degree of self-reactivity in both monoclonal and polyclonal B cell repertoires. We discuss new and historical evidence that self-reactivity is not just tolerated, but actively selected into the peripheral repertoire. We review recent progress in understanding how dual expression of the IgM and IgD BCR isotypes on mature naive follicular B cells tunes responsiveness to endogenous antigen recognition, and discuss how this may be integrated with other features of clonal anergy. Finally, we discuss how expression of Nur77 itself couples chronic antigen stimulation with B cell tolerance.

Optimal Development of Mature B Cells Requires Recognition of Endogenous Antigens.

It has long been appreciated that highly autoreactive BCRs are actively removed from the developing B cell repertoire by Ag-dependent receptor editing and deletion. However, there is persistent debate about whether mild autoreactivity is simply tolerated or positively selected into the mature B cell repertoire as well as at what stage, to what extent, under what conditions, and into which compartments this occurs. In this study, we describe two minor, trackable populations of B cells in B1-8i Ig transgenic mice that express the VH186.2 H chain and recognize a common foreign Ag (the hapten 4-hydroxy-3-nitrophenylacetyl) but differ in L chain expression. We use the Nur77-eGFP reporter of BCR signaling to define their reactivity toward endogenous Ags. The less autoreactive of these two populations is strongly counterselected during the development of mature B1a, follicular, and marginal zone B cells. By genetically manipulating the strength of BCR signal transduction via the titration of surface CD45 expression, we demonstrate that this B cell population is not negatively selected but instead displays characteristics of impaired positive selection. We demonstrate that mild self-reactivity improves the developmental fitness of B cell clones in the context of a diverse population of B cells, and positive selection by endogenous Ags shapes the mature B cell repertoire.

Nur77 Links Chronic Antigen Stimulation to B Cell Tolerance by Restricting the Survival of Self-Reactive B Cells in the Periphery.

Nur77 (Nr4a1) belongs to a small family of orphan nuclear receptors that are rapidly induced by BCR stimulation, yet little is known about its function in B cells. We have previously characterized a reporter of Nr4a1 transcription, Nur77-eGFP, in which GFP expression faithfully detects Ag encounter by B cells in vitro and in vivo. In this study, we report that Nur77 expression correlates with the degree of self-reactivity, counterselection, and anergy among individual B cell clones from two distinct BCR transgenic mouse models but is dispensable for all of these tolerance mechanisms. However, we identify a role for Nur77 in restraining survival of self-reactive B cells in the periphery under conditions of competition for a limited supply of the survival factor BAFF. We find that Nur77 deficiency results in the progressive accumulation of self-reactive B cells in the mature repertoire with age and is sufficient to break B cell tolerance in VH3H9 H chain transgenic mice. We thus propose that Nur77 is upregulated in self-reactive B cells in response to chronic Ag stimulation and selectively restricts the survival of these cells, gradually pruning self-reactivity from the mature repertoire to impose a novel layer of peripheral B cell tolerance.

tRNA binding, structure, and localization of the human interferon-induced protein IFIT5.

Interferon-induced proteins, including the largely uncharacterized interferon-induced tetratricopeptide repeat (IFIT) protein family, provide defenses against pathogens. Differing from expectations for tetratricopeptide repeat (TPR) proteins and from human IFIT1, IFIT2, and IFIT3, we show that human IFIT5 recognizes cellular RNA instead of protein partners. In vivo and in vitro, IFIT5 bound to endogenous 5'-phosphate-capped RNAs, including transfer RNAs. The crystal structure of IFIT5 revealed a convoluted intramolecular packing of eight TPRs as a fold that we name the TPR eddy. Additional, non-TPR structural elements contribute to an RNA binding cleft. Instead of general cytoplasmic distribution, IFIT5 concentrated in actin-rich protrusions from the apical cell surface colocalized with the RNA-binding retinoic acid-inducible gene-I (RIG-I). These findings establish compartmentalized cellular RNA binding activity as a mechanism for IFIT5 function and reveal the TPR eddy as a scaffold for RNA recognition.

Structural and biochemical analyses of Mycobacterium tuberculosis N-acetylmuramyl-L-alanine amidase Rv3717 point to a role in peptidoglycan fragment recycling.

Peptidoglycan hydrolases are key enzymes in bacterial cell wall homeostasis. Understanding the substrate specificity and biochemical activity of peptidoglycan hydrolases in Mycobacterium tuberculosis is of special interest as it can aid in the development of new cell wall targeting therapeutics. In this study, we report biochemical and structural characterization of the mycobacterial N-acetylmuramyl-L-alanine amidase, Rv3717. The crystal structure of Rv3717 in complex with a dipeptide product shows that, compared with previously characterized peptidoglycan amidases, the enzyme contains an extra disulfide-bonded ß-hairpin adjacent to the active site. The structure of two intermediates in assembly reveal that Zn(2+) binding rearranges active site residues, and disulfide formation promotes folding of the ß-hairpin. Although Zn(2+) is required for hydrolysis of muramyl dipeptide, disulfide oxidation is not required for activity on this substrate. The orientation of the product in the active site suggests a role for a conserved glutamate (Glu-200) in catalysis; mutation of this residue abolishes activity. The product binds at the head of a closed tunnel, and the enzyme showed no activity on polymerized peptidoglycan. These results point to a potential role for Rv3717 in peptidoglycan fragment recycling.

Large-scale mutational analysis identifies UNC93B1 variants that drive TLR-mediated autoimmunity in mice and humans.

Nucleic acid-sensing Toll-like receptors (TLR) 3, 7/8, and 9 are key innate immune sensors whose activities must be tightly regulated to prevent systemic autoimmune or autoinflammatory disease or virus-associated immunopathology. Here, we report a systematic scanning-alanine mutagenesis screen of all cytosolic and luminal residues of the TLR chaperone protein UNC93B1, which identified both negative and positive regulatory regions affecting TLR3, TLR7, and TLR9 responses. We subsequently identified two families harboring heterozygous coding mutations in UNC93B1, UNC93B1+/T93I and UNC93B1+/R336C, both in key negative regulatory regions identified in our screen. These patients presented with cutaneous tumid lupus and juvenile idiopathic arthritis plus neuroinflammatory disease, respectively. Disruption of UNC93B1-mediated regulation by these mutations led to enhanced TLR7/8 responses, and both variants resulted in systemic autoimmune or inflammatory disease when introduced into mice via genome editing. Altogether, our results implicate the UNC93B1-TLR7/8 axis in human monogenic autoimmune diseases and provide a functional resource to assess the impact of yet-to-be-reported UNC93B1 mutations.

Nur77 Is Upregulated in B-1a Cells by Chronic Self-Antigen Stimulation and Limits Generation of Natural IgM Plasma Cells.

B-1a cells are a unique population of innate-like B cells with a highly restricted and self-reactive BCR repertoire. Preimmune "natural" IgM produced by B-1a-derived plasma cells is essential for homeostatic clearance of cellular debris and forms a primary layer of protection against infection. In this study, we take advantage of a fluorescent reporter of BCR signaling to show that expression of the orphan nuclear hormone receptor Nur77 is upregulated under steady-state conditions in self-reactive B-1a cells in response to chronic Ag stimulation. Nur77-deficient mice exhibit elevated natural serum IgM (but not IgG) and marked expansion of IgM plasma cells of B-1a origin. Moreover, we show that Nur77 restrains the turnover of B-1a cells and the accumulation of immature IgM plasma cells. Thus, we identify a new critical negative-regulatory pathway that serves to maintain B-1a cells in a quiescent state in the face of chronic endogenous Ag stimulation.

Tonic LAT-HDAC7 Signals Sustain Nur77 and Irf4 Expression to Tune Naive CD4 T Cells.

CD4+ T cells differentiate into T helper cell subsets in feedforward manners with synergistic signals from the T cell receptor (TCR), cytokines, and lineage-specific transcription factors. Naive CD4+ T cells avoid spontaneous engagement of feedforward mechanisms but retain a prepared state. T cells lacking the adaptor molecule LAT demonstrate impaired TCR-induced signals yet cause a spontaneous lymphoproliferative T helper 2 (TH2) cell syndrome in mice. Thus, LAT constitutes an unexplained maintenance cue. Here, we demonstrate that tonic signals through LAT constitutively export the repressor HDAC7 from the nucleus of CD4+ T cells. Without such tonic signals, HDAC7 target genes Nur77 and Irf4 are repressed. We reveal that Nur77 suppresses CD4+ T cell proliferation and uncover a suppressive role for Irf4 in TH2 polarization; halving Irf4 gene-dosage leads to increases in GATA3+ and IL-4+ cells. Our studies reveal that naive CD4+ T cells are dynamically tuned by tonic LAT-HDAC7 signals.

Subfamily-specific adaptations in the structures of two penicillin-binding proteins from Mycobacterium tuberculosis.

Beta-lactam antibiotics target penicillin-binding proteins including several enzyme classes essential for bacterial cell-wall homeostasis. To better understand the functional and inhibitor-binding specificities of penicillin-binding proteins from the pathogen, Mycobacterium tuberculosis, we carried out structural and phylogenetic analysis of two predicted D,D-carboxypeptidases, Rv2911 and Rv3330. Optimization of Rv2911 for crystallization using directed evolution and the GFP folding reporter method yielded a soluble quadruple mutant. Structures of optimized Rv2911 bound to phenylmethylsulfonyl fluoride and Rv3330 bound to meropenem show that, in contrast to the nonspecific inhibitor, meropenem forms an extended interaction with the enzyme along a conserved surface. Phylogenetic analysis shows that Rv2911 and Rv3330 belong to different clades that emerged in Actinobacteria and are not represented in model organisms such as Escherichia coli and Bacillus subtilis. Clade-specific adaptations allow these enzymes to fulfill distinct physiological roles despite strict conservation of core catalytic residues. The characteristic differences include potential protein-protein interaction surfaces and specificity-determining residues surrounding the catalytic site. Overall, these structural insights lay the groundwork to develop improved beta-lactam therapeutics for tuberculosis.